Adults with perfect grammar keep a foreign accent because:

1. Adult learners nearly always keep an identifiable foreign accent, even some with perfect grammar. 2. A possible explanation is that pronunciation, or phonology, is susceptible to the critical period. 3. Pronouncing speech sounds relies on neuromuscular function. 4. Adults are past the prime age for learning new neuromuscular functions. 5. Vocabulary, learned consciously, shows no critical period in second-language learning. 6. So the explanation is that pronunciation is tied to age. **Why the other options fail:** - Adults can listen carefully and still keep an accent. - Vocabulary has no strict cut-off. - The stem says the grammar is perfect, so late grammar learning cannot be the cause. _Source: Wikipedia, "Critical period hypothesis", section "Second-language acquisition" (revision 1375020650)_